1. Field of the Invention
The present invention relates to a semi-transmissive liquid crystal display device which displays images by using a backlight in a dark environment and by utilizing reflection of outside light in a bright environment, and to a method for manufacturing the same.
2. Description of the Prior Art
A liquid crystal display device has advantages in that the device is thin and light, and the device can be driven at low voltage and uses much less power. Thus, the liquid crystal display device is widely used in various electronic devices. Particularly, an active matrix liquid crystal display device, in which a TFT (thin film transistor) is provided as a switching element for each picture element, is as excellent as a CRT (cathode ray tube) in terms of display quality. Thus, the active matrix liquid crystal display device is widely used in a display of a television, a personal computer or the like.
A general liquid crystal display device has a structure in which liquid crystal is sealed between two substrates disposed so as to face each other. A TFT, a picture element electrode and the like are formed on one of the substrates, and a color filter, a common electrode and the like are formed on the other substrate. Hereinafter, the substrate on which the TFT, the picture element electrode and the like are formed will be called a TFT substrate, and the substrate disposed so as to face the TFT substrate will be called a counter substrate. Moreover, the structure including liquid crystal sealed between the TFT substrate and the counter substrate will be called a liquid crystal panel.
There are three types of liquid crystal display devices, including: a transmissive liquid crystal display device which uses a backlight as a light source and displays images by using light transmitted through a liquid crystal panel; a reflective liquid crystal display device which displays images by utilizing reflection of outside light (natural light or lamplight); and a semi-transmissive liquid crystal display device which displays images by using a backlight in a dark environment and by utilizing reflection of outside light in a bright environment.
In the semi-transmissive liquid crystal display device, a transmission region where an amount of transmitted light is controlled, and a reflection region where an amount of reflected light is controlled are usually provided in one picture element. In the transmission region, light passes through a liquid crystal layer only once. On the other hand, in the reflection region, light passes through the liquid crystal layer twice (back and forth). Thus, in order to effectively utilize both of the transmitted light and the reflected light, it is required to match phase difference conditions of the liquid crystal layer between the reflection region and the transmission region. The phase difference conditions of the liquid crystal layer are indicated by the product of a refractive index difference (a difference in refractive index between extraordinary light and normal light) An of the liquid crystal layer and a thickness d of the liquid crystal layer. In principle, a refractive index difference Δ n of the liquid crystal layer or a thickness d of the liquid crystal layer in the reflection region is set at half a refractive index difference Δ n of the liquid crystal layer or a thickness d of the liquid crystal layer in the transmission region. Accordingly, light emitted toward the face of the panel from the reflection region and light emitted toward the face of the panel from the transmission region can be set in the same polarization state. Thus, efficiency in use of light is maximized. As one utilizing such a technology, for example, Japanese Patent Laid-Open No. Hei 11 (1999)-242226 describes a semi-transmissive liquid crystal display device which has a thinner liquid crystal layer in a reflection region than that in a transmission region, and has orientation films oriented in different directions between the reflection region and the transmission region.
Incidentally, in the case of displaying moving images on a liquid crystal display device, if a response speed of liquid crystal is slower than write time (normally, about 17 ms) of a display signal, an afterimage phenomenon occurs since orientation of liquid crystal molecules cannot follow the display signal. In order to avoid the afterimage phenomenon, it is known that setting the liquid crystal orientation to an OCB (optically controlled birefringence) mode is effective. In the liquid crystal display device of the OCB mode, a birefringence index phase difference of a liquid crystal layer is controlled by switching the liquid crystal molecules in a bend orientation state. Since a switching speed in the bend orientation is sufficiently faster than a switching speed in twist orientation or splay orientation, the liquid crystal molecules follow the display signal to be oriented. Thus, the afterimage phenomenon is avoided.
The bend orientation is realized by disposing two substrates so as to face each other, the substrates being processed so as to have the same tilt direction of liquid crystal molecules on a liquid crystal interface. If a pretilt angle of the liquid crystal molecules in the vicinity of the interface of the liquid crystal layer is less than 45°, the liquid crystal molecules are more stable in terms of energy when the molecules are subjected to the splay orientation. Thus, it is required to shift the liquid crystal molecules from the splay orientation to the bend orientation by applying a bias voltage to the liquid crystal layer.
Japanese Patent Laid-Open No. Hei 11 (1999)-7018 describes a liquid crystal display device in which a high pretilt angle region is partially provided, which gives a pretilt angle larger than its surrounding by 100 or more to liquid crystal molecules. In this liquid crystal display device, a region subjected to the bend orientation in an initial state, or a region likely to be shifted from the splay orientation to the bend orientation is provided. Thus, shift to the bend orientation is allowed to easily occur.
Japanese Patent Laid-Open No. Hei 11 (1999)-133429 discloses a transmissive liquid crystal display device which gives a pretilt angle of a specific direction to liquid crystal on an interface of a substrate, by irradiating an ultraviolet ray 68 from the specific direction onto a surface of the substrate on which a vertical orientation film 69 is formed, as shown in schematic views of FIGS. 1A and 1B. In this transmissive liquid crystal display device, among alkyl chains 70 protruding in random directions from the vertical orientation film, the alkyl side chains 70 protruding in a direction parallel to an irradiation direction of the ultraviolet ray 68 are left. Accordingly, liquid crystal molecules are tilted to the direction.
FIG. 3 of Japanese Patent Laid-Open No. 2002-207227 discloses a semi-transmissive liquid crystal display device which has a liquid crystal layer set thinner in a reflection region than in a transmission region, drives liquid crystal in an OCB mode in which orientation is changed in the transmission region between splay orientation and bend orientation, and drives the liquid crystal in an R-OCB mode exhibiting hybrid orientation in the reflection region. In this semi-transmissive liquid crystal display device, the principal axis of liquid crystal molecules in the liquid crystal adjacent to an orientation film is processed to be parallel to a surface of the orientation film. Also in the case of using a vertical orientation film, the liquid crystal molecules on an interface of the orientation film are set to be approximately parallel to the surface of the orientation film by ultraviolet irradiation.
In the above-described liquid crystal display device of Japanese Patent Laid-Open No. Hei 11 (1999)-242226, the phase difference conditions in the transmission region and the reflection region are allowed to match by setting the liquid crystal layer in the reflection region to be thinner than that in the transmission region. However, there is no reference made to a panel configuration and a manufacturing method in the case where the OCB mode is used as the liquid crystal orientation. Thus, there is a drawback that response characteristics of the liquid crystal molecules are not sufficient, and an afterimage phenomenon occurs. Moreover, the orientation direction of the liquid crystal is significantly changed on the boundary between the transmission region and the reflection region. Thus, the orientation of the liquid crystal molecules in this portion becomes unstable. Consequently, there is a drawback that an effective aperture ratio is lowered.
The liquid crystal display device described in Japanese Patent Laid-Open No. Hei 11 (1999)-7018 is related to horizontal orientation. Thus, there is no reference made to the semi-transmissive liquid crystal display device of the OCB mode using vertical orientation. There are two methods to partially form a high pretilt angle region in a horizontal orientation region, including a method for partially forming a vertical orientation film after a horizontal orientation film is formed, and a method for using a mixed orientation film. However, in the former method, there is a problem that a process is complicated. In the latter method, there is a problem that an orientation film material is limited, and uniform film formation is difficult.
The transmissive liquid crystal display device described in Japanese Patent Laid-Open No. Hei 11 (1999)-133429 is related to the horizontal orientation. However, there is no reference made to the semi-transmissive liquid crystal display device. In the semi-transmissive liquid crystal display device, the liquid crystal layer has different optical path lengths between the reflection region and the transmission region. Thus, means for matching the phase difference conditions of the liquid crystal layer is required. Moreover, if the same pretilt angle is given by irradiating ultraviolet rays onto the reflection region and the transmission region, there arises a problem that optical compensation conditions in black display differ between the reflection region and the transmission region.
In the semi-transmissive liquid crystal display device described in Japanese Patent Laid-Open No. 2002-207227, the principal axis of the liquid crystal molecules in the liquid crystal adjacent to the orientation film is set to be parallel to the surface of the orientation film. Specifically, since the liquid crystal molecules on the interface of the orientation film are subjected to the horizontal orientation, it is difficult to form stable bend orientation. Moreover, in the case of using the vertical orientation film, the liquid crystal molecules on the interface of the orientation film are subjected to the horizontal orientation by the ultraviolet ray. Thus, there arises a similar problem. The stability of the bend orientation largely depends on a pretilt angle of the liquid crystal molecules. If the pretilt angle of the liquid crystal molecules is less than 45°, the splay orientation is likely to occur, and if the pretilt angle is 45° or more, the bend orientation is likely to occur. This is because stable orientation states in terms of energy are different depending on the pretilt angle.